Epoxy resins have been used in a wide variety of applications, for example, as primers for metals because of their superiority in corrosion resistance, chemical resistance and flexibility. Similarly, the corrosion-inhibiting effect of phosphoric acid has been well-known for a long time. The combination of epoxy and phosphoric acid into one resin system is also not novel. However, the water-dispersible property of epoxy phosphate ester resin has pointed to a new direction to make more environmental friendly coating products, such as water-based epoxy phosphate ester coating materials, commonly referred to low VOC epoxy phosphate ester resins. Since then, more and more epoxy phosphate ester coating resins have been reported, for example, U.S. Pat. No. 4,289,812 for a general background of such a teaching.
The preparations of epoxy phosphate ester resins have been disclosed in U.S. Pat. Nos. 4,164,487, 4,331,574 and 4,397,970 from late 70s to early 80s. Accordingly, many attempts have been made to balance between the hydrophobic and hydrophilic portions of polymer molecules in order to achieve the better water-dispersability and coating properties. Investigations are still being focused on two major areas: the first area is the ratio of phosphoric acid to the epoxy resin which can cause defects in the coating, and the second is the water solubility and the dispersion stability of the epoxy phosphate ester resins. For example, U.S. Pat. No. 5,389,704 indicates that the unreacted phosphoric acid remaining in the resin product causes defects of the coating film, such as blushing of the resultant coating. Additionally, Japanese Patent Publication Gazette No. Hei 1-055,299 teaches a process of preparing a water-dispersible epoxy phosphate ester resin. However, the resultant resins have unduly large particle size and poor water dispersion stability.
Specifically, the prior art epoxy phosphate ester resins have provided the general guideline of the ratio of phosphoric acid to the oxirane groups. To balance the hydrophobic portion and hydrophilic portion directly affects water-dispersibility, stability and coating properties. The hydrophilicity is usually measured by acid value (acid number). A resin with either too high or too low acid value can affect its water dispersibility, emulsion stability and final coating properties. In the case of such high molecular weight epoxy phosphate ester resins for example, the prior art suggested the acid value of epoxy phosphate ester resin should not be below 20. Otherwise, the water-dispersibility and aqueous stability of such resin become to a major issue.
Currently, commercially available epoxy phosphate ester resins for coatings are the solution type and the emulsion type. The solution type is a product of epoxy resin and phosphoric acid in an organic solvent or solvents. When an epoxy phosphate ester resin has been neutralized and dispersed into water, the resultant product is the well-known emulsion type. The advantage of the solution type method is that it provides coating formulations having the flexibility to select co-reactants even though such materials are hydrophobic in nature. However, the VOC requirements for such coatings demand that the non-volatile weight (NVW) of such solutions be within the range of 65-70% by weight. Needless to say, such resin coating systems have failed to address the need for continuously reduction of VOC contents to meet environmental concerns.
The viscosity of a high-solid (60% or higher) solution of high molecular weight epoxies with or without phosphorization, have been beyond any meaningful measurement. This has prevented the handling of such products on a production scale at room temperature or at elevated temperatures, which has resulted in limited usefulness and a high waste rate. Thus, the high cost of such coating formulations has prohibited their wide use in the industry.
Additionally, because of the high level of phosphate present in the resin system relative to the epoxide groups, the acidity of phosphate functions as a catalyst in combination with the high molecular weight epoxy and has resulted in coating compositions which gelled and sealed faster than the organic polymer. Accordingly, it was difficult to obtain dry film thickness without blistering of the coating film. See, for example, U.S. Pat. No. 7,087,663, wherein without the proper ratio of phosphoric acid and epoxy group, the uncontrolled increase in the degree of polymerization of the formed epoxy resin provides high molecular weight resins, which results in the gelation of the batch and an unusable resin coating material. This is the reason why the production of the epoxy-phosphoric acid adducts is so difficult. Thus, in view of the inadequacies of the prior works, it would be highly desirable to develop epoxy phosphate ester based resins which possess (a) lower amounts of organic solvent, (b) good water-thinnability and dispersible stability, and (c) superior coating properties.